Wireless networks based on a standard like IEEE 802.15.4 (/ZigBee), IEEE 802.11 (/Wi-Fi) or IEEE 802.15.1 (/Bluetooth) are commonly known. These operate in the 2400-2483.5 MHz ISM band (Industrial, Scientific and Medical) band. They are relatively low power short range systems (10-100 m) and can be interfered by other radios that are nearby and can cause high interference levels on nearby frequencies. This can be a constraint on attempts to reduce power consumption in wireless interfaces.
ZigBee for example is a low data rate (250 kbit/s) system with security features and enables a mesh network to be created ad hoc, to enable any device to reach more distant devices. In 2012 the ZigBee PRO Green Power feature was announced which allows ultra low power devices such as battery-less devices to securely join ZigBee PRO networks through a wireless interface which is designed to minimise the amount of power used. It is a more eco-friendly way to power ZigBee products such as sensors, switches, dimmers and many other devices. These devices can now be powered just by harvesting widely available, but often unused sources of small amounts of energy such as motion, light, vibration.
Devices can use energy harvesting to overcome the disadvantages of being mains powered or battery powered. Mains powering a device results in an installation cost, and it can only be used for non-mobile devices. The batteries of battery powered devices have a limited lifetime and the economic and environmental cost of regularly replacing batteries is not always acceptable.
Ultra Low Power (ULP) wireless nodes are characterized by the fact that they require zero-maintenance (no battery replacements), without the need to be mains powered. This can be achieved in principle in several ways. A first possibility is by making the battery lifetime larger than an expected or designed product lifetime by reducing the power consumption, such that the batteries last longer than the expected lifetime of the product. In this case the product lifetime has an upper bound set by the self-depletion of the battery.
A second possibility either instead of or together with first possibility is to use energy harvesting techniques. By reducing the power consumption, such that the required power/energy levels can be harvested from the environment. Energy harvesting is commonly regarded as deriving energy from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), and using or storing it for small, wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. ZigBee Green Power [2] is an extension to the ZigBee PRO networking stack [1] that allows incorporating ultra low power nodes—with a focus on energy harvested nodes—in a ZigBee PRO network. ZigBee Green Power enables ultra low power devices to use a wireless interface to complete a communication to a backbone network using typically hundreds of microJoules of energy. There are three type of nodes involved:
GreenPower Device
The GreenPower Devices (GPD) are a type of ultra low power wireless nodes. They typically generate the commands that should have an effect on the sink nodes (GPS) in the backbone network such as a ZigBee PRO network. One example is an energy harvesting switch (being a GPD) that generates a Toggle command, to toggle the state of a lamp (being a GPS) on the ZigBee PRO network. Because of these restrictions in energy, GPDs can only transmit the packets a limited number of times: typically they broadcast packets 3 times on a single channel. This channel is predefined, is configured using switches on the device itself, or can be negotiated during an initial commissioning procedure. In its simplest form the GPD can only transmit packets, and cannot receive any packet.
GreenPower Proxy
The GreenPower Proxies (GPP) are the nodes on the backbone network (ZigBee PRO network for example) that are mains powered and, when they are within radio range of the GPD, pick up the packets broadcast by the GPD, and deliver them to the sink nodes (GPS) over the ZigBee PRO backbone network. Since the GPD broadcasts its messages, these can be received by multiple GPPs, creating a form of redundancy in the network. The GPPs can fulfill an application level role at the same time.
Mains Powered GreenPower Sink
The GreenPower Sinks (GPS) are the nodes on the ZigBee PRO network that are mains powered and can have an application level entity which can be controlled by the GPD. For example, a lamp that is toggled by the energy harvesting switch at the GPD. The GPS does not need to be in radio range of the GPD, but has to be connected over the ZigBee PRO network with one or more proxy nodes (GPP) within radio range of the GPD. GPS nodes can often also act as GPP nodes directly.